1. Field of the Invention
The present invention relates to an alternator driven by an internal combustion engine, for example, and relates to the construction of a stator for an automotive alternator mounted to an automotive vehicle such as a passenger car or a truck, for example.
2. Description of the Related Art
As automotive engines have become quieter, noise reduction has become desirable in auxiliary engine machinery, and the need for magnetic noise reduction particularly in automotive alternators mounted to engines, for example, has been strong, but there has been a tendency for magnetic noise to increase more and more with increased output.
Magnetic noise is caused by a stator core resonating due to magnetic excitation arising in an air gap portion between facing portions of a stator and a rotor, and it is necessary to suppress vibrations in the stator which is the source thereof because these vibrations radiate as magnetic noise. Because the stator core is constructed by laminating thin plates and is provided with long slender teeth, rigidity is low, and the shape is particularly prone to vibration, and there have been limits to improving the rigidity of the stator itself without increasing the size or weight thereof. Thus, consideration has been given to damping vibration in the stator core by means of a stator winding which is another structural component of the stator. Specifically, the aim has been to absorb vibrations in the stator core by means of the spring and damping factors of the stator winding, but there have been manufacturing problems such as those described below, and it has been difficult to control the hardness of the stator winding, which dictates the spring factor.
Japanese Publication No. HEI 4-61584 discloses a method for manufacturing a stator by inserting a stator winding into a stator core formed with slots on an inner circumferential surface, insertion being made from an axial direction using a winding insertion device.
In this example, the stator winding, which is first constructed by forming conductors into a star shape, is forcefully inserted from an axial direction into the cylindrical stator core, and there is a risk at this stage that the conductors will interfere with each other, or interfere with the stator core, causing damage, besides which coil ends of the stator winding protruding from both axial end surfaces of the stator core are not uniform, preventing achievement of a desired hardness distribution in the coil ends, and since the mechanical load applied to the coil ends varies greatly in the insertion method in question, large discrepancies in the hardness distribution have arisen between the two coil ends.
Japanese Patent Laid-Open No. HEI 9-103052, which was filed by the present applicants, discloses a stator for an alternator in which the assembly operation in which the stator winding is inserted into the stator core is significantly improved compared to the above publication.
FIG. 18 is an overall perspective of the stator for the automotive alternator described in this patent laid-open. This stator 140 includes: a cylindrical stator core 150 composed of a laminated core formed with a number of slots 150a extending axially at an even pitch in a circumferential direction; and a stator winding 170 wound onto the stator core 150. The stator winding 170 is a three-phase alternating-current winding composed of an a-phase alternating-current winding portion 170a, a b-phase alternating-current winding portion 170b, and a c-phase alternating-current winding portion 170c, and is constructed by bundling round wires in which a copper surface is coated with insulation. Moreover, Oa, Ob, and Oc are a-phase, b-phase, and c-phase output wires and Na, Nb, and Nc are a-phase, b-phase, and c-phase neutral-point lead wires.
In the stator 140 of the above construction, the a-phase alternating-current winding portion 170a, the b-phase alternating-current winding portion 170b, and the c-phase alternating-current winding portion 170c, which are first formed in a flat shape, are installed in a parallelepiped laminated body 183 so as to be stacked one on top of another, as shown in FIG. 19, and then, the stator 140 is prepared by bending the laminated body 183 into a cylindrical shape by means of a forming device (not shown) and welding core abutting portions 184.
Although the stator 140 for an automotive alternator shown in FIG. 18 significantly improves the assembly operation in which the stator winding is inserted into the stator core 150, the stator winding 170 is constructed by winding flexible round wires, and the round wires in a first coil end 190a and a second coil end 190b protruding from both axial end surfaces of the stator core 150 and not aligned, are nonuniform, and interfere with each other, and like the stator described in Japanese Publication No. HEI 4-61584, achievement of the desired hardness distribution in the coil ends 190a and 190b has not been possible. Furthermore, since mechanical loads are randomly applied to the coil ends 190a and 190b during formation of the cylindrical shape after the stator winding 170 has been inserted into the laminated body 183 because the round wires in the first coil end 190a and the second coil end 190b are not aligned and interfere with each other, the desired hardness distribution could not be achieved in the coil ends 190a and 190b. 
Thus, since it has been difficult to achieve the desired hardness, the rigidity of the coil ends 190a and 190b at both axial ends of the stator core 150 has been different, and not only has it not been possible to sufficiently suppress vibrations in the stator core 150, but the vibrations have even been exacerbated by the vibrational phases of the coil ends 190a and 190b. 
The present invention aims to solve the above problems and an object of the present invention is to provide a low-noise alternator by enabling the achievement of the desired hardness distribution in the coil ends, and suppressing vibrations in the stator core using this hardness distribution.
To this end, according to the present invention, there is provided an alternator comprising:
a rotor for forming north-seeking (N) and south-seeking (S) poles alternately about a rotational circumference; and
a stator comprising:
a stator core surrounding the rotor; and
a polyphase stator winding installed in the stator core,
the stator core being formed with a number of slots extending axially at a predetermined pitch in a circumferential direction,
the polyphase stator winding being provided with stator winding sub-portions in each of which a conductor is wound so as to alternately occupy an inner layer and an outer layer in a slot depth direction within the slots at intervals of a predetermined number of slots, the conductor folding back outside the slots at axial end surfaces of the stator core to form a first coil end and a second coil end,
the first coil end and the second coil end comprising a number of extended portions constituted by the conductors being lined up circumferentially, and
the first coil end and the second coil end having a hardness distribution in which hardness varies in an axial direction, the hardness distribution being the same in the first coil end and the second coil end.